1. Field of the Invention
The present invention relates to a control device and a control method for an internal combustion engine.
2. Description of the Related Art
Typically, in an internal combustion engine, such as an automobile engine the timing that yields maximum engine output is set as the ignition timing; and, when knocking occurs, the ignition timing is retarded to lower the temperature of the combustion chamber and suppress knocking.
Furthermore, with an internal combustion engine, as demand for engine output increases, a greater amount of fuel and air must be fed into the combustion chamber in order to fulfill the increased demand. Due to this, the opening amount of a throttle valve, which is provided in the intake passage, is adjusted according to the engine output request to adjust the amount of intake air taken into the internal combustion engine.
However, in recent years, in addition to adjusting the intake air amount by adjusting the opening amount of a throttle valve, the intake air amount is also adjusted by varying the maximum lift and the duration of an intake valve. Japanese Patent Application Publication 2001-263015 describes a mechanism for varying the maximum lift and duration. In the described variable valve operating mechanism, the maximum lift and the duration of the intake valve are varied in mutual synchronization; in other words, increasing the maximum lift of the valve also increases the duration of the valve. With adjustment of the intake air amount by driving the variable valve operating mechanism in this manner, target values for the maximum lift and duration are calculated based upon the amount of the engine output request. The variable valve operating mechanism is driven to bring the maximum lift and the duration towards these target values, and, thereby, the intake air amount is adjusted towards a value that corresponds to the engine output request.
The target values are, for example, increased as the engine output request increases. By changing the maximum lift and duration towards the target values, which vary in this manner, the intake air amount is increased as the engine output request increases. Furthermore, when calculating the target values, not only is the amount of the engine output request taken into account, but also the actual amount of intake air at that time. Due to this, the maximum lift and duration, which are changing towards their target values, are caused to attain appropriate values for obtaining the requested air intake amount.
When adjusting the intake air amount by varying the maximum lift and duration by driving the variable valve operating mechanism, during low load operation when the demand for engine output is small, such as during idling operation or the like, as the maximum lift and the duration decreases, the valve closing timing of the intake valve approaches a timing before the intake bottom dead center (“BDC”). In this state, when the engine output request increases because of acceleration or the like, the target values for the maximum lift and the duration are increased based upon the amount of this engine output request. Since the calculation of the target values is performed by taking into account the actual intake air amount, which increases progressively and gradually, accordingly the target values are gradually increased corresponding to the change of this actual intake air amount. Moreover, by driving the variable valve operating mechanism to gradually increase the maximum lift and the duration to the target values in response to a demand for increased engine output, it is possible to obtain an intake air amount which corresponds to the amount of the engine output request.
However, when the maximum lift and the duration are gradually increased from the low load operational state, along with this increase of the duration, the valve closing timing of the intake valve gradually changes from closing before the intake BDC to closing after the intake BDC. Here, when the intake valve closes at approximately the intake BDC, the actual compression ratio increases, which frequently causes knocking to occur and leads to the retarding of the ignition timing. Accordingly when, along with increase of the duration of the intake valve, the valve closing timing of the intake valve changes gradually from closing before the intake BDC to closing after the intake BDC, there is a high likelihood that knocking will occur, so that the ignition timing is retarded in order to suppress such knocking. Consequently, the increase of the engine output may be delayed, due to the retarding of the ignition timing.